In gravure printing, ink is filled in fine concave portions (cells) formed in a surface of a cylindrical plate cylinder (gravure cylinder), and while extra ink is scraped by a doctor, the plate cylinder is pressed against a medium to be printed (paper and the like), to thereby transfer the ink in the cells to the medium to be printed. Depending on the degree of the depth of the cell, the gradation or shading of the ink is expressed. Plate-making in gravure printing is performed by forming the cells in the surface of the gravure cylinder, and this has been conventionally performed by mechanical engraving. In recent years, however, from the viewpoint of high-definition printing, productivity improvement, or the like, laser plate-making, in which direct exposure of plate-making information (digital data of characters, images, and the like) is performed by an X-Y scanning type laser exposure apparatus using a semiconductor laser, has become the mainstream. In the laser plate-making, a photosensitive material film coated on the surface of the gravure cylinder rotating at high speed is subjected to exposure with use of semiconductor laser light having a wavelength of, for example, 830 nm, which is optically modulated by the plate-making information, and then subjected to development and etching, to thereby form a surface hardening film such as a chromium plated film. The laser plate-making is particularly suitably used in a so-called computer to plate (CTP) system, which performs plate-making by outputting the digital data of the plate-making information, such as characters and images, directly onto a plate without output to a film. Further, the laser plate-making is not only used in gravure plate-making, but also used in various types of plate-making such as offset plate-making and flexo plate-making. The inventors of the present invention have already developed a laser plate-making system in which all steps of this laser plate-making are fully automated, and have gained a high popularity (for example, Patent Document 1 and the like).
Currently, as a laser exposure method for laser gravure plate-making, a method with a resolution of about 3,200 dots per inch (dpi) is in widespread use. In this case, one pixel is expressed by about 7.92 μm2. It can be said that this resolution of 3,200 dpi provides sufficient performance in the general printing field, for example, printing of normal books, magazines, catalogs, and packaging films. However, a higher resolution is required in a case where, in the field of manufacture of an electronic component, such as a printed circuit board, a liquid crystal display, and a plasma display, which has a possibility as an application field, exposure of various circuit patterns is performed by an X-Y scanning type laser exposure apparatus instead of performing one-shot exposure or step exposure with use of an overlapped mask film as in the conventional case, or in a case of special printing for prevention of forgery of banknotes and the like.
A conventional laser exposure method with a resolution of 3,200 dpi in a sub-scanning direction and 3,200 dpi in a main scanning direction is described with reference to FIG. 9.
Referring to FIG. 9, description is made of a method of a case where exposure is performed with respect to a cylinder for gravure plate-making, which has a photosensitive film coated thereon, with use of a conventional laser exposure apparatus including a laser head portion. The laser head portion includes a laser light source for oscillating laser light, a light modulation portion for spatially arraying and partitioning the laser light by a plurality of control signals to obtain a plurality of laser beams, a projection optical portion for performing reduced size projection of the arrayed laser beams exiting from the light modulation portion, and scanning means for scanning the laser beams with respect to the photosensitive film. In the example of FIG. 9, 208 laser beams are used, and each laser spot has a square shape and sized so that the height is about 7.9 μm and the width is about 7.9 μm.
Under a state in which the cylinder having the photosensitive film coated thereon is rotated, the laser head portion is scanned in a spiral manner to perform exposure.
First, in a first rotation of the cylinder, a laser spot array having a predetermined length is formed for exposure on the photosensitive film by the 208 laser beams.
In a second rotation of the cylinder, a laser spot array is sub-scanned based on the following expression (1) so that beams to be applied are superimposed only on the 208th beam applied in the first rotation, and the beams are applied. In this case, only a region corresponding to the 208th beam applied in the first rotation is subjected to superimposed exposure.Pitch 25.4/3.200×207=1,643 mm  (1)
Next, in a third rotation of the cylinder, beams are applied so as to be superimposed on the 208th beam applied in the second rotation. In this case, a region corresponding to the 208th beam applied in the first rotation is subjected to superimposed exposure.
As described above, laser exposure with a resolution of 3,200 dpi×3,200 dpi is performed.
Further, recently, in order to further increase the resolution, there has been developed a laser exposure apparatus which generates a rectangular laser spot in which the height of the laser spot is approximately half of 7.9 μm, which is the size of the conventional laser spot. With such a laser exposure apparatus, laser exposure with a resolution of 3,200 dpi×6,400 dpi is possible.
Such a laser exposure method with a resolution of 3,200 dpi in the sub-scanning direction and 6,400 dpi in the main scanning direction is described with reference to FIG. 10.
Referring to FIG. 10, description is made of a method of a case where exposure is performed with respect to a cylinder for gravure plate-making, which has a photosensitive film coated thereon, with use of a conventional laser exposure apparatus including a laser head portion. The laser head portion includes a laser light source for oscillating laser light, a light modulation portion for spatially arraying and partitioning the laser light by a plurality of control signals to obtain a plurality of laser beams, a projection optical portion for performing reduced size projection of the arrayed laser beams exiting from the light modulation portion, and scanning means for scanning the laser beams with respect to the photosensitive film. In the example of FIG. 10, 208 laser beams are used, and each laser spot has a rectangular shape and sized so that the width is about 7.9 μm and the height is half the width.
Under a state in which the cylinder having the photosensitive film coated thereon is rotated, the laser head portion is scanned in a spiral manner to perform exposure.
First, in a first rotation of the cylinder, a laser spot array having a predetermined length is formed for exposure on the photosensitive film by the 208 laser beams.
In a second rotation of the cylinder, beams are applied based on the following expression (1) so as to be superimposed only on the 208th beam applied in the first rotation. In this case, only a region corresponding to the 208th beam applied in the first rotation is subjected to superimposed exposure.Pitch 25.4/3,200×207=1.643 mm  (1)
Next, in a third rotation of the cylinder, beams are applied so as to be superimposed on the 208th beam applied in the second rotation. In this case, a region corresponding to the 208th beam applied in the first rotation is subjected to superimposed exposure.
As described above, laser exposure with a resolution of 3,200 dpi×6,400 dpi is performed.
Recently, further increase in resolution has been demanded, but even though the height direction of the laser spot can be decreased as described above, developing a laser exposure apparatus which generates a laser spot with a further decreased size in a width direction has been quite difficult.
Further, a scan line method using individually drivable single stripe laser diodes is proposed (Patent Document 2).
However, it is preferred to achieve high resolution with use of the existing apparatus in terms of cost and the like.
In view of this, as a result of diligent studies, the inventors of the present invention have found a laser exposure method which is capable of achieving high resolution with use of the existing apparatus, which is covered in this proposal.